With the spread of information gathering and commercial transaction through web sites, problems such as virus infection and fraud caused by web access have rapidly increased. An example of this type of problem includes a problem in that if users access spoofed web sites (in other words, fake web sites) spoofing membership web sites or web sites of large companies, the spoofed web sites bring virus and cause virus infection, and a problem of abuse of personal information (so-called phishing) is caused by the spoofed web sites causing the users to mistake them for real web sites and inducing the users to input personal information such as a credit card number. Accordingly, a technology for preventing these problems has been proposed (for example, the technology disclosed in Patent Literature 1, and the like).
Patent Literature 1 discloses a technology of determining whether or not a string representing a URL is included in an email text with respect to an email that has not passed a content check for junk email or an email delivered from a source when the flow rate aggregated for each source IP address exceeds a predetermined threshold, and when the URL is included, registering the string in a database and discarding the email including the text with the string. Generally, in many cases, a person who opens a web site with malicious intent carries out an advertisement such as a distribution of large amounts of emails in which the URL of the web site is written in the email texts in order to lure victims to the web site. The technology disclosed in Patent Literature 1 is to prevent the spread of such an email with an advertising purpose and to prevent the occurrence of virus infection and fraud.